1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive pulse width modulating a voice coil motor using model reference current feedback.
2. Description of the Prior Art
A disk drive typically employs a voice coil motor (VCM) comprising a voice coil which interacts with permanent magnets to rotate an actuator arm about a pivot. A head is connected to a distal end of the actuator arm to actuate it radially over the surface of a disk in order to “seek” the head to a target data track. Once the head is positioned over the target data track, the VCM servo system performs a “tracking” operation wherein the head is maintained over a centerline of the data track while writing data to or reading data from the disk.
Conventionally the VCM has been driven using an H-bridge amplifier operating in a linear mode which leads to inefficient power dissipation since the transistors are driven with constant current. Driving the H-bridge amplifier in a pulse width modulated (PWM) mode increases the efficiency by driving the transistors in a switching mode (on-off) so that power dissipation occurs only when the transistors are switched on. FIG. 1 shows a prior art transconductance amplifier configuration for driving the VCM in a PWM mode using current feedback. The VCM comprises a voice coil 2 which has an intrinsic inductance (L) and resistance (R). The voice coil 2 is driven by an H-bridge amplifier comprising driver switches 4A–4D. A sense resistor R1 is connected in series with the voice coil 2, and an amplifier 6 amplifies the voltage across the sense resistor R1 to generate a voltage 8 representing the amplitude of current flowing through the voice coil 2.
The feedback loop established through amplifier 6 turns the voltage driver into a current driver, accomplishing the change from a voltage amplifier into a transconductance amplifier (which turns a voltage command into a current output). The voltage 8 representing the actual voice coil current is subtracted at node 10 from a voltage command u(k) 12 representing a desired voice coil current. The resulting voltage at node 10 is amplified by a high gain error amplifier 14 that generates a voltage command 16 applied to a first input of comparators 18A and 18B. A signal generator 20 generates a triangle waveform 22 applied to a second input of the comparators 18A and 18B. The comparators 18A and 18B generate PWM signals 24A and 24B having a duty cycle proportional to the current command 16. The PWM signals 24A and 24B are applied to switch control 26 which controls the driver switches 4A–4D in order to control the voltage applied to the voice coil 2. Resistor R4 and capacitor C1 in the feedback path between the current command 16 and the input voltage of error amplifier 14 provide lead compensation to compensate for the lag caused by the L/R time constant of the voice coil 2.
There are several drawbacks associated with driving a VCM in a PWM mode as illustrated in FIG. 1. For example, the various analog components of the error amplifier 14 increase the complexity and cost of the VCM driver circuitry. In addition, the periodic operation of the PWM mode introduces additional lag into the transconductance loop which decreases the loop bandwidth. The lead network provided by resistor R4 and capacitor C1 helps compensate for the additional lag, but the lead network must be designed conservatively to prevent instability due to the voice coil resistance drifting as the temperature fluctuates. The PWM lag can also be reduced by increasing the frequency of the PWM signals 24A and 24B, but this reduces the efficiency advantage of operating in a PWM mode. Due to these drawbacks, the VCM has been driven in a PWM mode using the configuration of FIG. 1 only during low bandwidth portions of the seek waveform. During short seeks and tracking operations, the H-bridge amplifier has been driven in a conventional linear mode so that the bandwidth can be increased without losing stability.
There is, therefore, a need to reduce the cost and increase power efficiency of the VCM driver circuitry in a disk drive.